Throughout this application various publications are referenced by full citations within parentheses. The disclosures of these publications in their entireties are hereby incorporated by reference in this application in order to more fully describe the state of the art to which this invention pertains.
Pharmacological studies, and more recently gene cloning, have established that multiple receptor subtypes exist for most, if not all, neurotransmitters. The existence of multiple receptor subtypes provides one mechanism by which a single neurotransmitter can elicit distinct cellular responses. The variation in cellular response can be achieved by the association of individual receptor subtypes with different G proteins and different signalling systems. Further flexibility is provided by the ability of distinct receptors for the same ligand to activate or inhibit the same second messenger system.
Individual receptor subtypes reveal characteristic differences in their abilities to bind a number of ligands, but the structural basis for the distinct ligand-binding properties is not known. Physiologists and pharmacologists have attempted to specify particular biological functions or anatomical locations for some receptor subtypes, but this has met with limited success. Similarly, the biochemical mechanisms by which these receptors transduce signals across the cell surface have been difficult to ascertain without having well-defined cell populations which express exclusively one receptor subtype.
Dopamine receptors have been classified into two subtypes, D.sub.1 and D.sub.2, based on their differential affinities for dopamine agonists and antagonists, and their stimulation or inhibition of adenylate cyclase (for reviews, see Kebabian, J. W. and Calne, D. B. (1979), Nature 277, 93-96; Creese, I., Sibley, D. R., Hamblin, M. W., Leff, S. E. (1983), Ann. Rev. Neurosci. 6, 43-71; Niznik, H. B. and Jarvie, K. R. (1989), Dopamine receptors. in "Receptor Pharmacology and Function", eds. Williams, M., Glennon, R., and Timmermans, P., Marcel Dekker Inc., New York, pp. 717-768). The D.sub.1 receptor of the central nervous system is defined as an adenylate cyclase stimulatory receptor. The location of the prototypic D.sub.1 receptor is the bovine parathyroid gland, where dopamine agonists stimulate cAMP synthesis via adenylate cyclase, accompanied by parathyroid hormone release. Dopamine-stimulated adenylate cyclase activity and parathyroid hormone release are sensitive to both GTP and cholera toxin. This suggests that the D.sub.1 receptor is associated with a G.sub.S guanine nucleotide binding protein. The D.sub.2 receptor, in contrast, inhibits adenylate cyclase activity, and appears to be the primary target of most neuroleptic drugs (Niznik, H. B. and Jarvie, K. R. (1989). Dopamine receptors, in "Receptor Pharmacology and Function", eds. Williams, M., Glennon, R., and Timmermans, P., Marcel Dekker Inc., New York, pp. 717-768). The prototypic D.sub.2 receptor has been characterized in the anterior pituitary where it is associated with the inhibition of release of prolactin and alpha-melanocyte stimulating hormones. Recent work has shown that several different D.sub.1 and D.sub.2 receptor subtypes may be present in the mammalian nervous system (Andersen, P. H., Gingrich, J. A., Bates, M. D., Dearry, A., Falardeau, P., Senogles, S. E., and Caron, M. G. Trends in Pharmacolog. Sci. 11: 231 (1990)), which would suggest that a family of different proteins with pharmacological properties similar to the classically defined D.sub.1 and D.sub.2 receptors may exist.
Neuroleptics, in addition to their use as drugs to treat severe psychiatric illnesses, are high affinity ligands for dopamine receptors. Butyrophenones such as haloperidol and spiperone are antagonists specific for the D.sub.2 receptor, while the recently developed benzazepines such as SCH-23390 and SKF-38393 are selective for the D.sub.1 receptor (Niznik, H. B. and Jarvie, K. R. (1989), Dopamine receptors, in "Receptor Pharmacology and Function", eds. Williams, M., Glennon, R., and Timmermans, P., Marcel Dekker Inc., New York, pp. 717-768). High affinity D.sub.1 and D.sub.2 selective ligands have conclusively distinguished these receptors and made feasible characterization of the receptors in the central nervous system and peripheral tissues with radioligand binding techniques. Two types of dopamine receptors, designated D.sub.A1 and D.sub.A2, have been identified in the cardiovascular system and are similar in their pharmacological characteristics to the brain D.sub.1 and D.sub.2 receptors (Niznik, H. B. and Jarvie, K. R. (1989), Dopamine receptors, in "Receptor Pharmacology and Function", eds. Williams, M., Glennon, R., and Timmermans, P., Marcel Dekker Inc., New York, pp. 717-768). D.sub.A1 receptors have been described in renal, mesenteric, splenic, coronary, cerebral, and pulmonary arteries and vascular beds, where dopamine elicits relaxation of vascular smooth muscle. Activation of cardiovascular D.sub.A1 receptors appears to stimulate adenylate cyclase activity. D.sub.A2 receptors appear to be localized on preganglionic sympathetic nerve terminals that mediate inhibition of norepinephrine release. The molecular relationships among dopamine D.sub.1, D.sub.A1, D.sub.2, and D.sub.A2 receptors are unknown.
The need for improved selectivity in the leading D.sub.1 drug class, the benzazepines (e.g. SKF-38393, SCH-23390 and SCH-23982) recently became apparent when the strong cross-reactivity of these drugs with the serotonin 5-HT.sub.2 receptor family was uncovered. The 5-HT.sub.2 and 5-HT.sub.1C receptors display affinities ranging from 0.2 to 24 nM for SCH-23390 and SCH-23982 (Nicklaus, K. J., McGonigle, P., and Molinoff, P. B. (1988), J. Pharmacol. Exp. Ther. 247, 343-348; Hoyer, D. and Karpf, A. (1988), Eur. J. Pharmacol. 150, 181-184)), raising the possibility that behavioral and pharmacological effects ascribed to these drugs may, in fact, arise from serotonergic receptor interactions.
The dopamine D.sub.1 receptors belong to a family of receptors which are distinguished by their seven-transmembrane configuration and their functional linkage to G-proteins. This family includes rhodopsin and related opsins (Nathans, J. and Hogness, D. S., Cell 34:807 (1983)), the .alpha. and .beta. adrenergic receptors (Dohlman, H. G., et al., Biochemistry 26:2657 (1987)), the muscarinic cholinergic receptors (Bonner, T. I., et al., Science 237:527 (1987)), the substance K neuropeptide receptor, (Masu, Y., et al., Nature 329:836 (1987)), the yeast mating factor receptors, (Burkholder, A. C. and Hartwell, L. H., Nucl. Acids Res. 13:8463(1985); Hagan, D. C., et al., Proc. Natl. Acad. Sci. USA 83:1418 (1986)); Nakayama, N. et al., EMBO J. 4:2643 (1985)), and the oncogene c-mas, (Young, et al., Cell 45:711 (1986)). Each of these receptors is thought to transduce extracellular signals by interaction with guanine nucleotide-binding (G) proteins (Dohlman, H. G., et al., Biochemistry 26:2657 (1987); Dohlman, H. G., et al., Biochemistry 27:1813 (1988); O'Dowd, B. F., et al., Ann. Rev. Neurosci., in press).
The D.sub.2 receptor was recently cloned by Civelli and colleagues (Bunzow, J. R., Van Tol, H. H. M., Grandy, D. K., Albert, P., Salon, J., Christie, M., Machida, C. A., Neve, K. A., and Civelli, O. (1989), Nature 336: 783-87). This event was soon followed by the discovery of an alternatively spliced form (termed D.sub.2A, D.sub.2long, D-2.sub.in, or D.sub.2(444)) that contains an additional 29 amino acids in the third extracellular loop of this receptor (Eidne, K. A. et al. (1989), Nature 342: 865; Giros, B. et al. (1989), Nature 342: 923-26; Grandy, D. K. et al. (1989), Proc. Natl. Acad. Sci. USA 86: 9762-66; Monsma, F. J. et al. (1989), Nature 342: 926-29; Chio, C. L. et al. (1990), Nature 343: 266-69; Stormann, T. M. et al. (1990), Mol. Pharmacol. 37: 1-6). A second dopamine receptor has been cloned which exhibits significant homology to the D.sub.2 receptor, both in amino acid sequence (75% transmembrane region identity) and in pharmacological properties (Sokoloff, P. et al. (1990), Nature 347: 146-51). This new receptor, termed D.sub.3, is encoded by an intron-containing gene. Unlike the D.sub.2 receptor, however, alternatively spliced isoforms of this receptor. have yet to be observed. The D.sub.3 receptor has been shown to serve both as an autoreceptor and as a postsynaptic receptor, and has been localized to limbic areas of the brain (Sokoloff, P. et al. (1990), Nature 347: 146-51). Finally, an intronless gene, quite different in sequence and gene structure from the other two dopamine receptor genes, has been isolated and identified as a D.sub.1 dopamine receptor subtype (Sunahara, R. K. et al. (1990), Nature 347: 80-83; Zhou, Q.-Y. et al. (1990), Nature 347: 76-80; Dearry, A. et al. (1990), Nature 347: 72-76; Monsma, F. J. et al. (1990), Proc. Natl. Acad. Sci. USA 87: 6723-27). This D.sub.1 receptor is predominantly expressed in the rat striatum and olfactory tubercles, and has been shown to couple to stimulation of adenylate cyclase activity (Dearry et al. (1990) supra; Monsma et al. (1990) supra; Sunahara et al. (1990) supra; Zhou et al. (1990) supra. Available data on the G protein-coupled receptor superfamily suggests that the D.sub.1 receptor does not exhibit strong sequence homologies to the D.sub.2 receptor or the D.sub.3 receptor. In general, G protein-coupled receptors of the same neurotransmitter family exhibit closest structural homology to other family members that use the same second messenger pathway. For example, examination of the physiological second messenger pathways of the serotonergic, muscarinic and adrenergic receptors has led several researchers to the conclusion that these receptors can be classified into structurally homologous subtypes that parallel their second messenger pathways (Bylund, D. B. (1988), Trends Pharmacol. Sci. 9, 356-361; Peralta, E. G., Ashkenazi, A., Winslow, J. W., Ramachandran, J., and Capon, D. J. (1988), Nature 334, 434-437; Liao, C.-F., Themmen, A. P. N., Joho, R., Barberis, C., Birnbaumer, M., and Birnbaumer, L. (1989), J. Biol. Chem. 264, 7328-7337; Hartig, P. R. (1989), Trends Pharmacol. Sci. 10, 64-69)). Interestingly, those receptors that couple to activation of adenylate cyclase appear quite distinct in structure from those that inhibit this enzyme activity.
Pharmacological and physiological data have emerged indicating the presence of further diversity within this receptor family. A D.sub.1 receptor that stimulates phosphoinositide (PI) hydrolysis in rat striatum has been described (Undie, A. S., and Friedman, E. (1990), J. Pharmacol. Exp. Ther. 253: 987-92) as well as an RNA fraction from the same tissue that causes dopamine-stimulated PI hydrolysis and intracellular calcium release when injected into Xenopus oocytes (Mahan, L. C. et al. (1990), Proc. Natl. Acad. Sci. USA 87: 2196-2200). In addition, two populations of peripheral D.sub.1 receptor have been described based on differential sensitivity to sulpiride and several other compounds (Andersen, P. H. et al. (1990), Eur. J. Pharmacol. 137: 291-93). Finally, pharmacological differences exist within different D.sub.1 receptor tissues that couple to adenylate cyclase-coupled D.sub.1 receptors. Biochemical and pharmacological data suggest further diversity in both the D.sub.1 and D.sub.2 receptor populations and indicate that additional dopamine receptor clones remain to be discovered (Andersen et al. (1990) supra).